1. Field of the Invention
The present invention relates to a control system for a belt-type continuously variable transmission of the type that includes a drive pulley which is variable in groove width; a driven pulley which is variable in groove width; an endless belt wound around both of the pulleys; a ratio control valve for controlling a gear ratio by increasing or decreasing a hydraulic pressure supplied to at least one of oil chambers for the pulleys; and a solenoid valve means which is controlled to supply a control hydraulic pressure to the ratio control valve.
2. Description of Related Art
In a known control system for a belt-type continuously variable transmission, the ratio control valve is controlled between a LOW position and an OD position by a single solenoid capable of being duty-controlled, thereby increasing or decreasing the hydraulic pressure supplied to at least one of the oil chambers of the drive pulley and the driven pulley to change the gear ratio.
Such known control system for the belt-type continuously variable transmission suffers not only from a problem that it is difficult to effect a fine precise control, because the gear ratio is controlled from the LOW value to the OD value by the single solenoid valve, but also is accompanied by the following disadvantage: when the solenoid valve fails or has an ON trouble or an OFF trouble, the gear ratio is held at the LOW or OD value and, as a result, it is difficult to start the vehicle, or an over-revolution of the engine is generated.